1. Field of the Invention
This invention relates to a flywheel device for a prime mover, used in a drive system e.g. of an automotive vehicle, for transmitting the torque of the prime mover while reducing torque variation and torsional vibrations.
2. Description of the Related Art
Conventionally, a flywheel device of this kind has been disclosed e.g. in Japanese Laid-Open Patent Publication No. H10-196724 (Page 5, FIG. 4). The flywheel device is a unitary assembly disposed between an internal combustion engine and a clutch, which is composed of a first rotary mass member coaxially mounted on the crankshaft of the engine, a second rotary mass member connected to the input shaft of a transmission via a clutch, and a damper and a friction device disposed between the first and second rotary mass members.
The damper is provided for damping torsional vibrations of the crankshaft, and includes a plurality of coil springs arranged on the first rotary mass member at circumferentially spaced intervals. These coil springs are each connected to both of the first and second rotary mass members. As the crankshaft rotates, the rotation of the crankshaft is transmitted from the first rotary mass member to the second rotary mass member via the coil springs of the damper, and further via the clutch, if engaged, to the input shaft of the transmission. In this process, the first and second rotary mass members reduce variation in the torque of the engine, while the damper reduces torsional vibrations.
If the damper alone is provided, when the clutch is not engaged, e.g. during the start of the engine, and the engine rotates at a relatively low engine speed without load other than the flywheel device, torsional vibrations tend to occur between the two rotary mass members, which tends to apply large load particularly to joints where the flywheel device are connected with the crankshaft and other components. The friction device prevents the torsional vibrations from occurring between the two rotary mass members e.g. during the start of the engine, and is comprised of a friction shoe that is disposed outward of the damper in the radial direction of the crankshaft and has a width progressively reduced in a tapered manner as the friction shoe extends inward in the radial direction of the crankshaft, and a shoe-abutting portion that is formed as a sloping surface of the first rotary mass member and in contact with a tapered side surface of the friction shoe. The second rotary mass member is formed with a protruding portion that protrudes from an outer end thereof toward the first rotary mass member, and has a spring attached to the inside of the protruding portion. The friction shoe, which is urged radially inward by the spring, urges the shoe-abutting portion of the first rotary mass member and an engine-side surface of the second rotary mass member.
According to the flywheel device described above, the damper and the friction device are rotated together with the first and second rotary mass members as the engine rotates. In the process, when the engine speed is low, the urging force of the spring overcomes the centrifugal force acting on the friction shoe to generate frictional forces between the friction shoe and the two rotary mass members. The frictional forces act as rotational resistance between the two members. This prevents generation of torsional vibrations during low engine speed, which cannot be prevented by the damper alone. When the engine speed rises to a certain point, the centrifugal force acting on the friction shoe overcomes the urging force of the spring, so that the friction shoe is moved radially outward against the urging force of the spring to be disconnected from the first rotary mass member, causing the frictional forces to stop acting between the friction shoe and the two rotary mass members. This enables the damper to reduce the torsional vibrations of the crankshaft, and torque with the reduced torsional vibrations is transmitted to the input shaft. Thus, the variation in the torque and the torsional vibrations can be properly damped according to the engine speed, whereby the torque can be stably transmitted to the input shaft.
However, the conventional flywheel device proposed in Japanese Laid-Open Patent Publication No. H10-196724 suffers from the following problems: In the case of the friction device described above, to stably generate a required magnitude of frictional forces between the friction shoe and the two rotary mass members, it is necessary to secure a contact area which is large to some extent. For this reason, the friction device is arranged such that it extends over a relatively large length from a location corresponding to radially central points of the rotary mass members toward a location corresponding to the outer ends of the same. Further, since the rotational resistance between the two rotary mass members is changed by moving the friction shoe in the radial direction, it is necessary to secure a radial size for the stroke of motion of the friction shoe, so that the size of the friction device is increased particularly in the radial direction. Accordingly, when the flywheel device is constructed by using such a friction device, the size of the entire flywheel device is increased. Moreover, when the clutch is engaged, the second rotary mass member is pressed toward the first rotary mass member, whereby the friction shoe is sandwiched between the two rotary mass members, causing the resulting load to act on the friction device, which can vary the magnitude of the frictional forces generated thereby to make it impossible to obtain the required magnitude of frictional forces. This makes it impossible to ensure stable operation of the flywheel device, and stably transmit the torque of the engine to the input shaft of the transmission.